Good chance your diagnosis is correct, always on the assumption that whoever rebuilt it did it correctly and the electrical bits are up to spec. The dynamo can easily be tested in isolation, as Rex has mentioned.
In this test set up output from terminal D is fed back into the field coil, terminal F, increasing the field strength and so the dynamo output rises.....a meter or other load eg 12volt headlamp bulb connected between this D-F link and the dynamo body should show a rapidly rising output voltage.
Dynamos rely on a small amount of residual magnetism remaining in the field coil pole piece to generate an initial output, the old style electro/mechanical regulator feeds this back to the field coil, and by modulating this controls the dynamo output and balances this against the required electrical demand
With low or no residual magnetism to start the process as on any newly built unit or an old dynamo which has been "asleep" for years a little bit of help is required. This is what you do when you " polarise" the dynamo, powering the field coil directly from the battery, with a temporary link. This is also the process used to change a dynamo from negative to positive earth and vice versa.
Plenty on the Forum about this procedure, and also as much there as you need to fix the problem. If the dynamo gives an output in isolation then the fault lies with the regulator or the rest of the loom.
Swarfy.
